The organic compounds undergoing a reversible change of color on exposure to photic or thermal energy which are typical and best known are spiropyran derivatives and G. H. Brown: Photochromism (John Wiley & Sons, Inc., 1971), for one, can be consulted for a specific listing of such derivatives and their physical properties. However, when an attempt is made to exploit the hitherto-known spiropyran derivatives commercially in recording materials, they are found to have serious disadvantages. Thus, because the meta-stable species (ring opened isomers) are lacking in thermal stability in solutions as well as in polymeric binders, they tend to promptly revert to the ground state or fade out so that the chromic response at a necessary level cannot be sustained for a sufficient duration.
Meanwhile, as piezochromic compounds, there are known, inter alia, 9-(p-nitrophenylphenylmethylene)xanthene [J. Am. Chem. Soc., 79, 6020 (1957)], diflavine [J. Am. Chem. Soc., 80, 6312 (1958)], dehydrodianthrone [Chem. Ber., 100, 280 (1967), hexaphenylbiimidazolyl (Bull. Chem. Soc. Japan, 38, 685 (1965); ibid. 38, 2202 (1965); ibid, 43, 429 (1970)], tetraphenylvinyl dimer [Bull. Chem. Soc. Japan, 43, 1431 (1970)], phthalocyanine-cobalt complex [Journal of Synthetic Organic Chemistry, Japan, 30, 521 (1972)] and hydroxycarboxylic acid derivatives [Japanese Kokai Patent Publication No. 132857/1988], as well as spiropyranthiopyrans [Japanese Kokai Patent Publication No. 46079/1977] and spirobenzopyranoxadiazoline derivatives [Japanese Kokai Patent Publication No. 42880/1977], both of which are commonly referred to briefly as spiropyrans. However, many of these known compounds have several problems that must be overcome, viz. some require a substantial pressure for coloration or have only a low contrast of color between the colorless state and the colored state, while others are too labile, as very compounds, to be utilized as industrial materials and still others involve a complicated route for synthesis.
It is an object of the present invention to provide a piezochromic compound which insures a sufficient color density in a stable manner.
It is a further object of the invention to provide a piezochromic compound which can be intensely colored with a relatively low pressure to give a large contrast of color between the colored and colorless states and is superior in thermal stability.
It is a still further object of the invention to provide an expedient process for synthesizing such a piezochromic compound.